Before You Start / Safety
For NB cars, internal sill/rocker corrosion is a known risk area. This guide focuses on internal corrosion slowing, not structural rust repair.
Warning: “Spray a continuous coat and do not spray on parts of the brake, engine or exhaust system.” https://irl.sika.com/en/industry/automotive-aftermarket/protective-coatings/cavity-waxes/sikagard-6220.html
Warning: “Cavity wax should not be applied to exhaust systems as it will produce smoke and can be flammable.” https://supportdesk.kbs-coatings.com/255826-Cavity-Wax-Provides-Rust-Protection-in-Hard-to-Reach-Areas
If rust is already perforating structural metal, this is body-repair territory first.
Required Tools
- Cavity wax product with long wand/360° lance
- PPE (gloves, eye protection, mask/respirator as product label requires)
- Trim tools for access panels
- Flashlight/endoscope (helpful)
- Drain cleaning tools (non-sharp flexible probe)
- Absorbent cloths/cardboard for overspray control
Required Parts / Fluids
- Cavity wax (NB-compatible anti-corrosion product)
- Optional cavity access grommets/plugs if new access points are created
High-risk quote: “Very good creep capability … protects even hardly accessible areas and tight sheet intervals.” https://irl.sika.com/en/industry/automotive-aftermarket/protective-coatings/cavity-waxes/sikagard-6220.html
Model-specific notes (NB1 vs NB2)
- NB1/NB2 rust-prevention strategy is the same: keep drains clear and protect internal sill/rocker cavities.
- Access details vary by trim/interior and previous repair history.
Manual-reference quote: “1999-2001 Mazda Miata Service Repair Manual” and “2005 Mazda Miata Service Repair Manual” are listed in the Miata factory manual index. https://www.mellens.net/mazda/
Step-by-Step Procedure
1) Inspect and clean first (do not spray over packed debris)
Remove obvious debris and confirm water can exit drains before applying wax.
Community quote: “The problem with these cars is … water pools INSIDE the rockers.” https://www.reddit.com/r/Miata/comments/1od5p1b/.json
2) Identify access points
Use existing service/access holes where possible. If additional access is required, plan small, rust-protected, grommeted openings.
3) Prep surfaces and temperature
Follow product prep guidance (clean, dry where possible; loose scale removed from reachable areas).
High-risk quote: “Surfaces must be clean, dry and free of rust, dust and grease.” https://irl.sika.com/en/industry/automotive-aftermarket/protective-coatings/cavity-waxes/sikagard-6220.html
4) Apply cavity wax with lance while withdrawing wand
Apply evenly through cavities, especially sills/rockers, pillars, and boxed sections. Use slow wand withdrawal for full internal coverage.
High-risk quote: “In accordance with nearly all OEMs, 3M recommends applying at least three coats … to all unprotected metal surfaces.” https://www.3m.com/3M/en_US/collision-repair-us/featured-products/cavity-wax-plus/
5) Keep drain paths open
Do not flood lower drain exits shut.
High-risk quote: “When applying inside cavities, make sure that all drain holes remain open.” https://supportdesk.kbs-coatings.com/255826-Cavity-Wax-Provides-Rust-Protection-in-Hard-to-Reach-Areas
6) Re-check drainage and clean overspray
After cure/set per product spec, run controlled water and confirm outlet flow under the car.
Community confirmation quote: “Are your drains clogged?” https://www.reddit.com/r/Miata/comments/1qrerkl/.json
7) Plan periodic re-inspection
Internal wax protection is maintenance, not one-and-done forever. Recheck annually (or sooner in salted/wet climates).
Torque Specs / Capacities
- No universal torque values apply to cavity-wax application itself.
- If removing structural trims/brackets, use year/VIN FSM torque values for reassembly.
Verification / Post-service checks
- Drain holes visibly open and flowing
- No wax contamination on brakes/exhaust/hot surfaces
- No trapped water signs after rain/wash
- Rust progression monitored at each service interval
Uncertainty / Open Questions
- Source access remained uneven this run (some rustproofing pages blocked/404), so this guide relies on retrievable manufacturer/technical references and NB community evidence.
- Exact NB access-point maps vary by car condition/previous repairs; adapt to what is present without creating water traps.
Images
No reusable, clearly licensed NB-specific cavity-wax procedure images were added in this run.
Image Credits
- No images used.
Sources
- Mellens.net — Mazda Miata Factory Service Manuals. Retrieved 2026-03-14. https://www.mellens.net/mazda/
- 3M — Cavity Wax Plus & Wand Kit For Collision Repair. Retrieved 2026-03-14. https://www.3m.com/3M/en_US/collision-repair-us/featured-products/cavity-wax-plus/
- Sika — Sikagard®-6220. Retrieved 2026-03-14. https://irl.sika.com/en/industry/automotive-aftermarket/protective-coatings/cavity-waxes/sikagard-6220.html
- KBS Coatings — Cavity Wax Provides Rust Protection in Hard-to-Reach Areas. Retrieved 2026-03-14. https://supportdesk.kbs-coatings.com/255826-Cavity-Wax-Provides-Rust-Protection-in-Hard-to-Reach-Areas
- Bilt Hamber — Dynax-S50. Retrieved 2026-03-14. https://bilthamber.com/product/dynax-s50/
- Reddit r/Miata — NB rust on sill - from inside or surface rust? (JSON endpoint). Retrieved 2026-03-14. https://www.reddit.com/r/Miata/comments/1qrerkl/.json
- Reddit r/Miata — Prevent Rust NB Miata? (JSON endpoint). Retrieved 2026-03-14. https://www.reddit.com/r/Miata/comments/1od5p1b/.json